There are several types of fuel cells currently available, all with specific advantages and disadvantages. Currently, work has been intensifying in the area of methanol reformers, devices that can convert methanol and water into hydrogen and carbon dioxide. The hydrogen from such a device can be used to run a fuel cell. Typically, these reformers operate at 200-300° C., and produce several tenths of a percent carbon monoxide in their effluent stream. Proton exchange membrane (PEM) fuel cells typically operate at <85° C. At these temperatures, more than 100 ppm carbon monoxide in the fuel stream is typically poisonous to the anode catalyst of a fuel cell. In order to alleviate this condition, a preferential oxidizer, or PROX, is used to selectively oxidize carbon monoxide in the fuel stream to carbon dioxide, while leaving most of the hydrogen unreacted, before it reaches the fuel cell. The preferential oxidizer lowers the carbon monoxide levels to less than 100 ppm, yet this is still enough to poison the anode of the low temperature PEM fuel cell.
A fuel cell that operates at 80° C., while intrinsically a fairly efficient device, nonetheless liberates about 50% of the energy in the fuel stream as heat. At such temperatures, this waste heat is of low quality and cannot be used to drive the reforming reaction, which is endothermic. Typical proton exchange membranes, such as Nation, also require aggressive humidification for optimal ionic conductivity and peak performance. Supplying the fuel cell with both fuel and oxidant gas streams at near saturated levels increases system complexity. Also, a fuel cell is a device that creates water as product, while typically necessitating a tight operating window where conditions must be delicately balanced between saturation for optimum performance, while avoiding condensing conditions. which chokes off gas access to the electrodes and degrades performance.
Thus, a nominal fuel cell is preferably ideally suited, both thermally and chemically, for operation in conjunction with a methanol or other hydrocarbon fuel reformer. Phosphoric acid fuel cells (PAFCs) are well suited for these conditions, as they can operate at higher temperatures.
U.S. Pat. No. 6,833,204 to Hiroyuki Oyanagi et al, assigned to Honda Giken Kogyo Kabushiki Kasha, issued Dec. 21, 2004; as well as U.S. Pat. No. 6,703,152 and United States Patent Applications Pub. Nos. 2004/0009377, 2002/0012823, and 2002/0012822; provide the following state of technology information: “The phosphoric acid fuel cell has a power-generating cell which is provided with an electrolyte-electrode assembly comprising an anode electrode, a cathode electrode, and an electrolyte layer interposed between the both electrodes. The electrolyte layer is generally constructed such that pores of a porous silicon carbide member is impregnated with concentrated phosphoric acid (liquid electrolyte). However, another type of the electrolyte layer is also known, in which a membrane of basic polymer such as polybenzimidazole is impregnated with phosphoric acid or sulfuric acid (see U.S. Pat. No. 5,525,436). In the phosphoric acid fuel cell, a predetermined number of the power-generating cells are electrically connected in series with each other to provide a fuel cell stack which is accommodated in a container. When the phosphoric acid fuel cell is operated, at first, the hydrogen-containing gas is supplied to the anode electrode, and the oxygen-containing gas is supplied to the cathode electrode.
The hydrogen in the hydrogen-containing gas is ionized on the anode electrode in a manner as represented by the following reaction formula (A). As a result, the hydrogen ion and the electron are generated.2H2→4H++4e  (A)
The hydrogen ion is moved toward the cathode electrode via the electrolyte layer. On the other hand, the electron is extracted by an external circuit which is electrically connected to the anode electrode and the cathode electrode. The electron is utilized as the DC electric energy to energize the external circuit, and then it arrives at the cathode electrode. The hydrogen ion moved to the cathode electrode and the electron arrived at the cathode electrode via the external circuit cause the reaction as represented by the following reaction formula (B) together with the oxygen in the oxygen-containing gas supplied to the cathode electrode.O2+4H++4e→2H2O  (B)
The reaction according to the reaction formula (B) is slow as compared with the reaction formula (A). That is, the reaction represented by the reaction formula (B) constitutes the rate-determining step in the overall cell reaction of the phosphoric acid fuel cell.”